Strains of Theiler's murine encephalomyelitis virus (TMEV) are divided into two subgroups on the basis of varying biological activities. GDVII strain and other members of the GDVII subgroup are highly neurovirulent and cause an acute, lethal neuronal disease with no virus persistence, while DA strain and other members of the TO subgroup are less neurovirulent and produce a chronic demyelinating infection. In addition, suckling mouse brain-passed DA is more neurovirulent than tissue culture-passed DA. Our working hypothesis is that discrete regions of the TMEV genome are critical for the varying biological activities of the two TMEV subgroups and of the mouse brain-passed vs. tissue culture-passed DA; our basic goals are to define these regions through manipulations of TMEV cDNA and to clarify the mechanisms involved. The TMEV system is a valuable one for such studies because: the mouse is both the natural and experimental host; the virus is a relatively simple one; a large amount of molecular, structural, and immunological information is known about TMEV; chimeric cDNAs and recombinant viruses can be prepared in order to study the two molecularly similar TMEV subgroups that naturally induce very different diseases. We have generated infectious cDNAs from GDVII strain and DA strain RNAs and subsequently prepared chimeric GDVII/DA cDNAs and recombinant viruses. These studies have demonstrated that the GDVII 1B-2C coding region contains a major neurovirulence determinant; however, the full neurovirulence potential of a recombinant virus is only achieved if the GDVII sequence 5' to this coding region is additionally present. We hypothesize that key determinants are within the 5'-UTR to the P1 coding region, and that certain proteins (that may vary in amount in neural cells compared to other cells) bind to the 5'-UTR affecting neurovirulence. This grant seeks to identify the key neurovirulence determinants and the mechanisms involved, and to characterize and identify the role of the 5'-UTR binding proteins and their functional role. A knowledge of the determinants of TMEV neurovirulence and their mechanisms of action may elucidate mechanisms of motor neuron tropism and injury relevant to motor neuron disease.